So you can get a fairly aggressive CSS optimization scheme for basically free, and you can iteratively improve the quality of your atomic CSS as you learn more about plain CSS / semantic taxonomy practices. And meanwhile, you're still just writing bog standard CSS and Mithril templates. There's no compile-time shenanigans to dedupe styles, no extra library runtime, and CSS is actually CSS so if you server-render you stay on the browser's streaming rendering happy path instead of waiting blocked on scripts.

Now compare with what one would do with most frameworks: one would pick some CSS-in-JS library, get things done and over with, but probably learn nothing interesting in the process and have little idea of what sort of code is actually getting shipped to customers.